1. Field of the Invention
The present invention relates to a method of designing a collimator array device which is used as an optical path changing switch module, an optical demultiplexing filter module, or the like, in an optical communication system, and a collimator array device manufactured by the method.
In practice, the above-mentioned module is obtained by combining a collimator array device and an optical switch array, an optical demultiplexing filter, or the like. However, in the present application, the whole device is referred to as a collimator array device.
2. Description of the Related Art
A general collimator array device (four by four optical switch modules) is shown in FIGS. 1 and 2. In FIG. 1, reference mark 1 indicates an emitting side fiber array. Reference mark 2 indicates an emitting side planar microlens. Reference mark 3 indicates an optical switch array. Reference mark 4 indicates a receiving side microlens. Reference mark 5 indicates a receiving side fiber array. The emitting side fiber array 1 and the receiving side fiber array 5 are respectively formed by installing a plurality of single mode optical fibers 1a and 5a between two sheets of silicon substrates 6a and 6b. Lenses 2a and 4a, having diameters of about 250 μm, are respectively formed in the emitting side planar microlens 2 and the receiving side microlens 4. Mirrors 3a are provided per a pixel in an optical switch array 3.
Mirror 3a is made to reflect or transmit light by inserting the mirror of a minute size in the optical path or displacing therefrom, or by electrically varying the refractive index of the material for the front and the back of the reflection surface. Therefore, the channel can be changed between the emitting side fiber array 1 and the receiving side fiber array 5 by combining the reflection and the transmission of each of the mirrors 3a, arranged 4 by 4, of the optical switch array 3.
The laser beam emitted from the end surface of the single mode optical fiber 1a installed in the emitting side fiber array 1 is collimated by the lens 2a formed in the emitting side planar microlens 2. The optical path of the collimated laser beam is deviated by the optical switch array 3, and thereafter, the laser beam is converged by the lens 4a formed in the receiving side microlens 4. The converged laser beam is made incident upon the single mode optical fiber 5a installed in the receiving side fiber array 5.
In FIG. 1, by giving predetermined mirrors 3a to the optical path, the laser beam emitted from the optical fiber A1 is made incident upon the optical fiber B4, the laser beam emitted from the optical fiber A2 is made incident upon the optical fiber B2, the laser beam emitted from the optical fiber A3 is made incident upon the optical fiber B3, and the laser beam emitted from the optical fiber A4 is made incident upon the optical fiber B1.
A laser beam is Gaussian beam the intensity of which is large in the center portion and small in the periphery portion.
The feature of Gaussian beam is shown in FIG. 3. While the light emitted from the emitting side optical fiber 1a side is collimated by the emitting side lens 2a, converged by the receiving side lens 4a, and made incident upon the receiving side optical fiber 5a, the collimated light is not parallel, i.e., it has a beam waist of 2W1 width in the intermediate portion. In addition, it does not converge upon one point (focal point).
In order to reduce the insertion loss at the receiving side, it is important that the end surface of the receiving side optical fiber 5a be adjusted to accurately coincide with the beam waist position of the laser beam emitted from the receiving side lens 4a and that the mode field diameter of the receiving side optical fiber 5a and the width 2W2 of the beam waist of the laser beam incident thereupon be adjusted to coincide (coupling).
In order to accurately conduct such a coupling, it is necessary that the beam waist of the laser beam emitted from the lens 2a be positioned at half of the distance between the emitting side lens 2a and the receiving side lens 4a, i.e., the optical length L. In other words, it is necessary that the distance d1 between the end surface of the emitting side planar microlens and the beam waist be equal to L/2.
In a case where the optical path of a laser beam is deviated by an optical functional element such as an optical switch array or the like, the optical length L from the emitting side planar microlens through the receiving side planar microlens is varied. For example, in FIG. 1, if one edge of a pixel of the optical switch array is set at 1 mm (therefore, one edge of the array is 4 mm), and the distance between each of the emitting side planar microlens and the receiving side planar microlens and the optical switch array is set at 2 mm, the optical length of the laser beam emitted from the optical fiber A1 and incident upon the optical fiber B4 is 11 mm (8 mm+3 mm), which is the largest, and the optical length of the laser beam emitted from the optical fiber A4 and incident upon the optical fiber B1 is 5 mm (8 mm−3 mm) which is the smallest. The standard value (8 mm) indicates the mean length in this case.
As shown in FIG. 3, the laser beam passing from the emitting side lens 2a through the receiving side lens 4a has a beam waist. The position of the beam waist is determined by the distance d0 between the end surface of the emitting side optical fiber 1a and the emitting side lens 2a. Therefore, if the optical length L is different from the condition shown in FIG. 3, i.e., the position of the receiving side lens 4a is shifted to the left side or the right side in FIG. 3, the position of the beam waist 2W2 of the laser beam emitted from the receiving side lens 4a is varied, the position of the beam waist 2W2 is shifted from the end surface of the receiving side optical fiber 5a, and thereby the insertion loss is increased.
Further, if the materials for the fiber arrays 1, 5 and the planar microlenses 2, 4 are different, the linear expansivities thereof are also different. Therefore, in such a case, if thermal variation occurs, the core of the optical fiber is shifted from the center of the lens.